1. Field of Invention
The present invention describes a new and unique technique in which the Raman effect is amplified through the use of co-incident entangled biphotons and the resulting Raman signature is captured by quantum state transfer between entangled biphotons. The geometry of the system is selected to produce two monochromatic, coherent, polarized, quantum state entangled, exclusively or nearly exclusive co-incident photons beams through Spontaneous Parametric Downconversion (SPDC). One beam, the probe beam, is directed at the sample or target, while the other beam, the detector beam, is directed at a remote quantum state characteristic detector. Both the resultant conventional Raman backscatter spectrum and the changes in the quantum state characteristics of the entangled biphotons are analyzed to determine the sample's composition.
2. Background of the Invention
Raman effect involves the way in which light scatters off any surface. That is to say, when light of any wavelength impinges on a surface (or molecule), most of the scattered photons are elastically (or Rayleigh) scattered. That means that they leave with the same frequency (or wavelength) as the incident radiation. However, a small fraction of the scattered light (less than one in a thousand incident photons) is inelastically (or Raman) scattered at frequencies that differ from the incident frequency by a value determined by the molecular vibrations of the sample. This process can be thought of as being similar to a Two Photon Absorption (TPA) event. That is, in order for Raman scattering to occur two photons must strike a molecule at the same time. But unlike TPA, one of the photons is absorbed while the other is simultaneously re-emitted at a different frequency by either gaining or losing some energy from the molecule's vibrational energy state. In other words, Raman scattering creates a discrete molecular spectrum at frequencies corresponding to the incident frequency plus or minus the molecular vibrational/rotational frequency. A Raman spectrum is thus a plot of the intensity of scattered light as a function of frequency (or wavelength). By convention, Raman spectra are presented graphically with the wave numbers (reciprocal centimeters) along the horizontal axis and the abscissa representing intensity or energy.
Raman spectra have long been used to determine the structure of inorganic and biological molecules, including the composition of complex multi-component samples. Raman spectroscopy is considered to have many advantages as an analytical technique. Most strikingly, it provides vibrational spectra that act as a molecular fingerprint containing, unique, highly reproducible, detailed features, thereby providing the possibility of extremely selective molecular determinations.
As compared to other forms of analysis, the Raman approach is advantageous for several reasons:                1. Solid, liquid and gas states can be analyzed        2. Aqueous solutions present no special problems        3. No special pre-scanning preparation of the sample is necessary        4. The low frequency region is easily obtained        5. The device can be made inexpensive lightweight and portable        6. Scanning can be completely non invasive and non destructive        7. Scanning distance can be varied from millimeters to kilometers        
As discussed above, it is well known that conventional Raman scattering relies on the simultaneous, random co-incidence of 2 photons striking a sample to produce a Raman frequency shift. Since lasers produce high-density photon populations they greatly increase the chance of random simultaneous photon strikes. This in turn greatly improves the Raman signal intensity. But even with this improvement, it is still a major challenge for all of the Raman techniques to date to collect spectral information with sufficiently high signal-to-noise ratios to discriminate weak analyte signals from the intertwined background noise. This is especially true if it has to be done quickly and nondestructively.
Due to the incredible potential of Raman scattering as a tool for high-resolution molecular or chemical analysis prior inventors have developed various Raman scattering techniques. Each in their own way have improved upon the conventional Raman backscattering technique through amplification of the signal production, improved the signal detection or improved signal analysis. These techniques include:
Dispersive Raman: This is a technique where the Raman backscatter is processed through a optical grating to separate out each individual frequency peaks which are then simultaneously analyzed.
Fourier Transform (FT-R): This is a technique where the Raman backscatter signal is collected as a composite of frequency peaks. This composite spectrum is processed through a Fourier Transform algorithm to yield each individual frequency peak.
Resonance Raman (RR): Resonance Raman scattering occurs when the photon energy of the exciting laser beam matches that of an electronic transition of a chromophoric group within the system under study. Under these conditions bands belonging to the chromophore are selectively enhanced by factors of 103 to 105.
Coherent Anti-Stokes Raman Spectrum (CARS): This is a technique where the Raman backscatter signal is amplified by illuminating the specimen under investigation by two monochromatic light fields, called pump- and Stokes-signal. Similar to RR, their frequency difference is tuned to a Raman-active transition of the chemical compound of interest.
Surface Enhanced Raman Spectroscopy (SERS): Is a technique where Raman backscattering intensity of a sample is significantly enhanced when the target molecules are absorbed onto an electrochemically roughened surface.
Surface Enhanced Resonant Raman Spectroscopy (SERRS): This technique is the same as SERS except the frequency used for excitation of the sample is that of an electronic transition of a chromophoric group within the system under study.
Fourier Transform Surface Enhanced Raman Spectroscopy (FT-SERS): This is the same technique as SERS except the signal analysis is processed through a Fourier Transform algorithm.
Each of the above conventional techniques has been developed in an attempt to overcome the notorious low quantum efficiency of Raman scattering. In other words, very few inelastic scattering events occur in comparison to the number of elastic scattering events. Conventionally, in non-resonance Raman spectroscopy in order to double the efficiency of Raman scattering it is necessary to square the photon density. Unfortunately this can damage the sample. Therefore it is necessary to perform scans at either long integration times or high power densities to achieve acceptable signal-to-noise ratios.
The other forms of Raman scattering like, resonance and surface enhancement or the combination of both can significantly improve the sensitivity and selectivity of Raman measurements. However, these enhancements are not generally applicable to all analytes or to all samples.
All of the above techniques rely on excitation with conventional lasers and measure the reflected backscatter through convention detection. The embodiment of this invention intends to improve and extend each of these techniques by using twin entangled co-incident biphotons as the Raman backscattering excitation source. It also intends to measure the resultant Raman effect via detection and analysis of the backscatter characteristics through quantum state transfer between entangled biphotons as well as by conventional means.
Although not devised for Raman backscattering, previous inventors have recognized the potential for using twin biphotons for improving the signal production when scanning of objects. U.S. Pat. No. 5,796,477 discloses an “entangled-biphoton” microscope, for WF fluorescence microscopy.